Ion microprobe evidence for the mechanisms of stable isotope retrogression in high-grade metamorphic rocks

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doi: 10.1007/s004100050020
Authors:Holness, M. B.
Author Affiliations:Primary:
University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 118(4), p.356-364. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1995
Note:In English. 34 refs.; illus., incl. 1 table
Summary:An investigation was made of the effect of trace amounts of feldspar (Na and/or K) on dihedral angles in the quartz-H2O-CO2 system at 4 kbar and 450-1050°C. Quartz-quartz-H2O dihedral angles in feldspar-bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at T << 500°C. Above this T, they decrease with increasing T until the solidus. The final angle at the inception of melting is ∼ 65° for microcline-quartz-H2O andmicrocline-albite-quartz-H2O, and much less than 60° (the critical value for formation of grain-edge fluid channels in an isotropic system) for the albite-quartz-H2O system. CO2 was observed to produce a constant quartz-quartz-fluid dihedral angle of 97° in feldspar-bearing quartz aggregates at all T studied. Also examined were the dihedral angles for the two co-existing supersolidus fluids in quartz aggregates. In all systems the quartz-volatile fluid angle is >> 60°, whereas the quartz-melt angle is << 60°. Both supersolidus angles decrease with increasing T. The transition from nonconnected to connected porosity with increasing T observed in the quartz-albite-H2O system some tens of degrees below the solidus (termed a permeability transition), if a common feature of rocks near their melting points, will play an important role in controlling the permeability of high-grade rocks to aqueous fluids. [P.Br.]
Subjects:Feldspar group; Fluid phase; Framework silicates; High pressure; Melting; Metamorphic rocks; Migmatites; P-T conditions; Permeability; Pressure; Quartz; Silica minerals; Silicates
Abstract Numbers:95M/2907
Record ID:1995034547
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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